Optical finger navigation (OFN) sensors are used to sense relative movement of a user's finger. OFN sensors are mainly implemented using small lenses or optical imaging systems (lenses, prisms, and apertures). The current trend is to implement OFN sensors within devices that have relatively small form factors. However, the use of a small form factor results in design difficulties. For example, the requirements on the physical dimensions of the imaging system as a whole limit the overall size of the OFN sensors.
The source of light in many conventional sensors is incoherent illumination from a light-emitting diode (LED). Since the illumination source is incoherent, it is typically not possible to track the finger movement speckle patterns. Consequently, incoherent light sources are often used in combination with other optical elements (e.g., lenses, apertures, etc.). However, the use of these types of additional optical elements can increase the overall size of the OFN sensor, which is in conflict with the trend of reducing the overall size of devices that use OFN sensors.
Other conventional OFN sensors use coherent illumination, which facilitates the use of speckle patterns to track finger movement. The propagation of the coherent beam from different locations of a diffusing surface such as a tracking surface produces the speckle pattern on the detector even without the presence of any imaging optics. However, the use of coherent illumination is unsatisfactory in conventional thin OFN sensors, which have a total thickness of about 1 mm, because the resulting average speckle size is too small relative to the pixel pitch, or the size of the individual pixels, of the detector. The thickness of thin OFN sensors constrains the optical distance of the light path from the tracking surface to the pixel array and results in the relatively small speckles. As one example, for an optical distance of about 1 mm between the navigation surface and the detector array, the average speckle size is less than 1 μm for a circular illumination area with 1 mm diameter at an optical wavelength of 850 nm. If the detector array has a pixel pitch larger than 1 μm, then there is an averaging effect of the speckle pattern, which reduces the contrast of the speckle patterns. In general, the contrast is the ratio of standard deviation of the intensity on the detector surface over its mean. If the navigation device is capable of tracking based on a speckle pattern with a contrast of 10% or more, then the maximum size of the active area on each pixel is about 10 μm. However, this maximum pixel size is much smaller than commercially available and low cost sensors used in OFN devices. Conversely, if the same speckle pattern were detected by a detector array with larger pixels, then the averaging effect would lower the contrast below an acceptable level.